British Prime Minister Theresa May has defended her Brexit plan and told plotters getting rid of her as leader won't make the process of leaving the European Union "any easier".

She was speaking at the end of a tumultuous week in which she presented her draft agreement on how the UK would leave the EU, a deal that sparked multiple resignations from her government.

"These negotiations have been tough right from the start, but they were always going to get more difficult towards the end," May told Sky News on Sunday.

Theresa May said replacing her would not make dealing with the EU any easier, or change the "parliamentary arithmetic". (PA/AAP)

"Replacing me is not going to make the (Brexit) negotiations any easier and it won't change the parliamentary arithmetic," she added.

She defended her deal as "what's right for the people of this country" and "the national interest".

"That's what drives me and that's what I am being driven to deliver," she said, adding: "I think that's what this deal does."

With her own leadership believed to be under threat from rebellious MPs, a defiant May denied she had thought about quitting and reiterated that the UK will leave the EU on March 29, 2019.

At least 21 Conservative MPs have publicly submitted letters of no-confidence in May and demanded that she step aside. A total of 48 letters are needed to trigger a party leadership election that could topple her as prime minister.

The resignation of Brexit Secretary Dominic Raab has prompted 21 Conservative MPs to submit letters of no confidence to the PM. (PA/AAP)

In an interview with The Sunday Times, former Brexit secretary Dominic Raab - whose decision to quit the Cabinet triggered the resignation of several other government ministers - said May had allowed the UK to be "blackmailed and bullied" by the EU.

He added that a deal should be demanded that would allow the country to leave the Customs Union unilaterally.

"I don't think we should look like we're afraid of our own shadow. We need to be going out there and grasping opportunities," Raab told the paper.

"If we cannot close this deal on reasonable terms we need to be very honest with the country that we will not be bribed and blackmailed or bullied and we will walk away."

Raab later told the BBC he would vote against the Brexit deal in parliament, but had not submitted a letter of no confidence in the Prime Minister.

Meanwhile, Labour's Jeremy Corbyn, also speaking on Sky News, touted the possibility of a general election and claimed a second Brexit referendum was "an option for the future".

Corbyn said if May's Brexit agreement comes to Parliament in its current state the Labour Party would vote against it.

"We do not believe it serves the interests of the country therefore the government would have to go back to the EU and renegotiate."

The Labour leader, who voted Remain during the 2016 referendum, also admitted that he wasn't sure how he'd vote if there was a second referendum.

"I don't know how I would vote - what the options would be at the time," Corbyn said.

Also on Sunday, the Scottish National Party leader Nicola Sturgeon told the BBC her party would vote against the Brexit deal.

If Labour and other opposition MPs all vote in line with what their party leaders said Sunday, and Conservative rebels join them, the government could lose the parliamentary vote - throwing the Brexit process into further disarray.